Mike wrote to the "Consult-Ant" with a number of questions about ants. I am going to try to answer each one in a separate post. For the original list of questions and links to all answers, visit here.
Question 1) I'm interested in the ant's digestion process and its role in the colony. I've read that some ants, the fully mature ones that is, can't digest solid food due to their narrow waists. So is it true that the ants would take the solid food back to the colony and give them to their larvae where, there, it is digested and converted into a liquid form? I'm guessing that the larvae have the enzymes to digest the solid food. But does this process apply to ALL ants in terms of species?
I actually tackled this topic to some extent in my post about ant larvae, but I'll re-state some of the high points here.
Because the petiole is so narrow and constricted, ants do have a specialized digestive system. When an ant eats, the food goes into a special pouch called the infrabuccal pocket in its mouth, which acts like a food strainer. The infrabuccal pocket prevents large particles from continuing into the digestive system. Probably the size of the particle allowed through varies from species to species, but in the carpenter ant, Camponotus pennsylvanicus, particles larger than 100 microns are excluded from entering the alimentary canal . The liquid and tiniest particles that can move through the narrow petiole are sucked into a tube and pass through into a special reservoir in the gaster called the crop. The ant spits out the leftover bits that were too big to go through the food strainer. The rejected bits are called infrabuccal pellets.
Scientists have long known that the worker ants feed all solids to the larvae first for processing. The larvae were thought to chew up, swallow and predigest the food, using enzymes, as you guessed. The larvae then regurgitate it back to the workers to distribute throughout the colony.
Recently, however, researchers have shown that in one species of bigheaded ants (Pheidole) the workers actually place the food on the surface of the belly of the larvae in a special groove (larvae lay on their backs). The larvae spit out the enzymes onto the food, basically drooling on themselves. After a few hours, the workers come back and pick up the slime that results, feeding some of it to the larvae and taking some for themselves. According to videos of the larvae processing bits of fruit fly, the larvae very rarely sip any of the gooey liquid while the food is dissolving; they wait patiently until the food is done and let the worker ants feed them.
Probably the most surprising aspect of ant larvae is that not only do worker ants bring them food, but the larvae are often sources of food themselves. Certain species of ant larvae have special structures that allow the workers to access the internal body fluids (hemolymph), a sort of pump or "tap."
The so-called Dracula ants take things a step further. These rare ants get their name from the fact that they cut holes in the sides of the larvae and suck out hemolymph. Although this sounds pretty gruesome, the larvae survive having holes bitten into them and later become workers themselves.
Another odd behavior of this group is that the workers carry the larvae to their food and place them on it, rather than carrying the food to the larvae, as most other ants do. For example, instead of cutting up a caterpillar or millepede into chunks and carrying it into the nest to feed the larvae, Dracula ants carry the larvae out to the caterpillar. Once they have fed, the larvae become food themselves.
If you think about it, the adult ant's weird digestion system does work because adults don't need a lot of protein for growth, they mainly need carbohydrates for energy. The same is true for some birds. The adult birds may drink sap or nectar, but feed protein-rich insects to their growing chicks.
Anyone else have anything to add?
Gotwald, W.H, Jr. 1969. Comparative morphological studies of the ants, with particular reference to the mouthparts (Hymenoptera: Formicidae). Cornell University Agricultural Experiment Station. Memoir 408.
Hansen, L. D., and J. H. Klotz. (2005). Carpenter ants of the United States and Canada. Ithaca, NY: Comstock Publishing Associates. This book has an excellent chapter on ant morphology.
Keiichi Masuko (1986). Larval Hemolymph Feeding: A Nondestructive Parental Cannibalism in the Primitive Ant Amblyopone silvestrii Wheeler (Hymenoptera: Formicidae). Behavioral Ecology and Sociobiology, Vol. 19, No. 4: 249-255
Keiichi Masuko (1989). Larval Hemolymph Feeding in the Ant Leptanilla japonica by Use of a Specialized Duct Organ, the “Larval Hemolymph Tap” (Hymenoptera: Formicidae). Behavioral Ecology and Sociobiology, Vol. 24, No. 2: 127-132
Keiichi Masuko (2008). Larval stenocephaly related to specialized feeding in the ant genera Amblyopone, Leptanilla and Myrmecina (Hymenoptera: Formicidae). Arthropod Structure & Development Volume 37, Issue 2: 109-117
D. L. Cassill, J. Butler, S. B. Vinson and D. E. Wheeler (2005). Cooperation during prey digestion between workers and larvae in the ant, Pheidole spadonia. Insectes Sociaux Volume 52, Number 4: 339-343.